DIATOMS LSI 



larger now coll (''rojuvonosccnce"). Thoy gonorato ])y 

 tlio oscapo and union of tlio protoplasms of two contigu- 

 ous colls whose half-cells have separated, resulting in the 

 formation of one or two new and usuall}- much larger 

 cells. Small biciliate isogametes have been doubtfully 

 reported in some marine diatoms. 



260. There are two general kinds (orders) of Diatoms, 

 namely, the Round Diatoms (Eupodiscales) with the cells 

 mostly round in end view, and the Flat Diatoms 

 {Xaviculales) with the filaments flattened in end view. 



261. The Round Diatoms are mostly 



marine and fossil. The ends of the cells J . 1 , 1 .L.L 

 are usually marked radially with lines or (^ 



rows of dots, as in Melosira, Coscinodiscus, fig. 73.— a 

 Actinodiscus, etc. Some Round Diatoms Meioglr?!^"'"'"' 

 form long filaments (Melosira). 



262. The Flat Diatoms occur abundantly as fresh- 

 water, marine, and fossil plants. The ends of the cells 



(transection of the flat filament) are often 

 marked transversely or pinnately by dots or 

 lines. In many of our most common Flat 

 Diatoms (e.g. Naviculaccae) there is a me- 

 dian longitudinal slit (''raphe") in the end 

 FiS^D^at^n^ wall, which probabl}^ has to do with the mo- 

 Baciiiaria. ' ^-j-^^. cxhi))ited by thoso pUiuts (Par. 174). 



263. Origin of Zygophyceae. It may be assumed that 

 the plants of this phylum have been derived from other 

 filamentous plants, and that the adhesion of cell to coll, 

 and the consequent formation of a multicellular plant 

 body, had l)ocome a well established habit long before 

 the peculiarties arose which set them off as Zygophy- 

 ceae. We must search among the Confervoideae of the 

 pnH'oding phylum for the ancestral tyi)os from whioh the 

 Conjugate Algae may have descended. Such plants as 



